1. Field of the Invention
The present invention relates to a semiconductor device of high withstand voltage, which improves withstand voltage characteristics of its inactive region.
2. Description of the Related Art
A conventional semiconductor device, such as an N channel type MOS transistor, is formed in an elliptical shape. The semiconductor device includes an N type drain region disposed in its center region, an N type buffer region disposed circularly surrounding the N type drain region, and an N type drift region disposed circularly surrounding the N type buffer region. The semiconductor device also includes a P type well region disposed surrounding the N type drift region, and an N type source region disposed in each of linear regions of the P type well region. With this structure, it is intended to achieve an improvement in on-state withstand voltage characteristics of the device. This technology is described for instance in Japanese Patent Application Publication No. 2000-156495, pp. 6 and 7, and FIG. 1.
A conventional semiconductor device, such as an N channel type MOS transistor, is formed in an elliptical shape as previously mentioned. The semiconductor device includes a P type semiconductor substrate, a semiconductor layer formed on the P type semiconductor substrate, a P type body region formed on the semiconductor layer, and an N type source region and a P type contact region which are formed in the P type body region. The semiconductor device also includes a drain region formed on the semiconductor layer, and a field oxide film formed between the drain region and the body region. The semiconductor device further includes four P type floating field rings disposed circularly under the field oxide film. This technology is described for instance in Japanese Patent Application Publication No. 2005-93696, pp. 6 and 7, and FIGS. 1 and 2.
As mentioned above, the conventional semiconductor device, specifically the N channel type MOS transistor, is formed in the elliptical shape. The semiconductor device includes the P type well region in a circular form, which is formed in linear regions and round regions to be used as active regions and inactive regions, respectively. The conventional semiconductor device has a problem that it is difficult to achieve desired withstand voltage characteristics, since the inactive regions in particular, with the same structure as that of the linear regions, is prone to convergence of an electric field due to its round shape when the MOS transistor is off.
Moreover, the conventional N channel type MOS transistor includes the four P type floating field rings, which are circularly disposed between the drain region and the body region, in order to improve the withstand voltage characteristics of the round region. With this structure, the conventional MOS transistor has a problem that it is difficult to achieve desired current characteristics, since the P type floating field rings form barriers against the flow of free carriers (electrons) between the drain and source regions and thus lead to an increase in the on-state resistance value.
The conventional semiconductor device includes the field oxide film formed between the drain and source regions, as mentioned above. With this structure, the conventional semiconductor device has a problem that it is difficult to achieve desired current characteristics, since the field oxide film forms a barrier against the flow of free carriers (electrons) between the drain and source regions and thus leads to an increase in the on-state resistance value.